1. Field of the Invention
The present invention relates to a specimen information acquisition system.
2. Description of the Related Art
X-ray phase imaging is a method in which the phase change of X-rays caused by a specimen is detected to thereby acquire an image regarding the specimen on the basis of the detection result. There has been proposed an X-ray phase imaging method such as one described in Japanese Patent Application National Publication (Laid-Open) (Translation of PCT Application) No. 2010-502977 (corresponding to US2010/0054415). In this method, the amount of refraction of X-rays caused by a specimen is detected to thereby acquire information regarding the phase change of the X-rays by utilizing the fact that X-rays are refracted by the phase change thereof caused by a specimen.
The principal of this method will be briefly described. In the method, X-rays are, at first, spatially divided by a grating having blocking portions which block X-rays and transmitting portions which transmit X-rays therethrough. The thus divided X-rays become X-ray beams. The X-ray beams are directed to a specimen, and the X-ray beams which have passed through the specimen are detected by an X-ray detector (hereinafter, may just be referred to as a detector). This makes it possible to find out how far the positions of the respective X-ray beams incident on the detector are displaced by the specimen. Further, information regarding the refraction of the X-ray beams caused by the specimen can be obtained on the basis of the displacement amount. A specimen may be arranged between an X-ray source and a grating, and X-rays refracted by the specimen may be divided by the grating.
When performing X-ray phase imaging according to the above method, the phase detection sensitivity of an X-ray beam is generally improved when using an X-ray beam having a smaller width. This is because of that since the amount of refraction of an X-ray beam caused by a specimen does not depend on the width of the X-ray beam, the smaller the width of the X-ray beam is, the larger the displacement amount of the X-ray beam with respect to the width thereof becomes.
In order to make the width of an X-ray beam smaller, the size of an effective focal point (hereinafter, a focal point indicates an effective focal point) of an X-ray source should be reduced. However, in a common X-ray source, the focal point (hereinafter, also referred to as an X-ray focal point) thereof vibrates during the generation of X-rays. The vibration of the X-ray focal point leads to the increase of the apparent area of the X-ray focal point. Since the vibration of the X-ray focal point results from the expansion of an anticathode which is caused by heat generated when electrons are applied to the anticathode and the shrinkage thereof caused by cooling, the irregular deviation of a rotation axis which supports an rotating anticathode, the vibration of the body of the X-ray source, and the like, it is difficult to completely eliminate the vibration of the X-ray focal point. In the present specification, displacement of the X-ray focal point, the displacement having no particular period caused by the aged deterioration of the X-ray source and the like, is also regarded as the vibration. Further, when electrons are applied to the anticathode, the applied electrons are scattered inside the anticathode due to the interaction between atoms constituting the anticathode and the applied electrons. Since the scattered electrons cause unevenness in the luminance of the X-ray focal point, the smaller the size of the effective focal point becomes, the closer the intensity distribution becomes to a normal distribution from a rectangular distribution. In the present specification, a part of the X-ray focal point in which the luminance is low is referred to as a hem. The width of each of the X-ray beams incident on the detector becomes larger due to the influence of the hem. As a result, an overlapping portion is formed between adjacent X-ray beams. The overlapping between the adjacent X-ray beams incident on the detector hinders the detection of the displacement amount of each of the X-ray beams. The hem of the X-ray focal point also has no relation to the size of the effective focal point as with the vibration amount. Therefore, the smaller the size of the effective focal point becomes, the relatively larger the influence of the hem of the X-ray focal point on the width of each of the X-ray beams becomes.
Japanese Patent Application Laid-open No. 2011-200532 discloses an X-ray imaging system in which the application period of electrons applied to an anticathode is synchronized with the rotation period of the anticathode for reducing the increase of the apparent area of an X-ray focal point caused by the vibration of the X-ray focal point. Further, Japanese Patent Application Laid-open No. 2011-200532 discloses an X-ray imaging system including a shutter which is provided outside an X-ray source, and opens and closes at the same period as the rotation period of an anticathode.
However, although the X-ray imaging system disclosed in Japanese Patent Application Laid-open No. 2011-200532 solve the problem of vibration of the focal point, the vibration having the same period as the rotation period of the anticathode of the X-ray source, the influence by the scattering of the electrons applied to the anticathode is not taken into consideration.